In the case of turbomachines, conical screw connections are used in a fuel line as standard in order to make it possible, in particular, to realize a geometric arrangement of the fuel line on the outer side of the burner of the turbomachine without problems. In the case of the applicant's gas turbines, use is made, for example, of 24° conical screw connections conforming to ISO 8434-4.
In the presence of the high operating temperatures that typically arise in turbomachines, fuel lines, or conical screw connections comprised by such fuel lines, and the individual parts thereof are likewise exposed to relatively high temperatures and large temperature fluctuations. For this reason, it is normally the case that no soft material seals are used for such conical screw connections, because such seals would not be capable of withstanding the high temperatures, or excessively fast aging of such seals would occur. The conical screw connections that are thus typically used in the case of the fuel lines have a metallic sealing cone which is inserted into a likewise metallic screw connection piece with a form fit, wherein said sealing cone and screw connection piece may additionally be screwed together to form a sealed closure. A conical screw connection of said type is known for example from DE 10 2005 028 188 A1. The sealing action of the two metallic components which are in contact by way of the sealing surfaces is normally adequate in the newly constructed state. However, said sealing action deteriorates over the course of the operating duration, predominantly owing to the vibrations that occur during operation. Likewise, the sealing action deteriorates owing to actions arising from the assembly process, differential temperature expansions and insufficiently appropriate handling of the fuel lines, as a result of which the sealing action falls to below an admissible level after a certain period of usage.
Said deteriorated sealing action between the sealing cone and screw connection piece gives rise to leaks, which must be avoided. The prior art counteracts said leaks after a sufficiently long period of usage on the one hand through replacement of the sealing cone, by virtue of said sealing cone being cut off from the screw connection body which comprises it and a new sealing cone being welded onto the remaining screw connection body. To be able to ensure the sealing action of the welded connection between the new sealing cone and new screw connection body, nondestructive weld seam testing must be performed. Both the cutting of the sealing cone from the screw connection body and the replacement thereof with a new seal cone which must be welded on involve expenditure of work which necessitates a large amount of time and costs and material.
On the other hand, the prior art proposes, as described for example in DE 41 16 540 C2, that loose seals be placed into the screw connection piece, which seals are then compressed by the sealing cone and the screw connection piece in order to impart a sealing action. A problem with this approach is however the lack of accuracy with which such loose seals can be inserted into the screw connection pieces. Specifically, practice has shown that even a slightly eccentrically inserted seal leads to a non-uniform arrangement of the seal even in the compressed state, and thus an inadequate sealing action. Specifically in the case of fuel lines which are charged with high pressures, however, such an inadequate sealing action constitutes a safety risk, which must be avoided.